This invention relates to a method for producing a beam-like structural part with a light-weight core.
In the construction of a motor vehicle, which is the preferred use of the present invention, it is desirable to provide a light-weight structural beam that will absorb kinetic energy upon impact, consisting of a hollow metal member made from two or more component sections, for example, a channel and a cover plate, welded together. German Offenlegungsschrift No. 2 222 557 discloses the use of a light-weight core in the form of a foamed member to be placed in a hollow metal member after foaming in order to achieve a controlled absorption of kinetic energy by accordion buckling. That reference does not disclose, however, how the core of light material is connected to the hollow member so as to transmit force. Also, the method described therein is evidently limited to structural members having a shape which permits a prefabricated core of light structural material to be pushed into the hollow metal member from an open end, since the hollow member is apparently completed prior to insertion of the core. This is apparent because, if the hollow member were to be assembled by welding several component sections together only after insertion of the core of light-weight material, there would be a danger of damage to the core resulting from the heat applied during welding.
This difficulty is also present in the use of the method described in German Offenlegungsschrift No. 27 47 721, in which foam material is inserted into a plastic bag which was previously mounted in a hollow member and is intended to form adhesive bonds with the hollow member. In addition, this method is especially complicated where the hollow members have substantial length since, in that case, additional measures such as inflation with compressed air are required to expand the bag for introduction of the foam material.
European Patent No. 0 247 239 describes a method of producing a coatable light-weight structural part, in particular for vehicles, in which a metal sheet is first coated with a cross-linked layer of a polyester, epoxide or polyurethane compound, after which the metal sheet is formed into the desired shape. Layers of fiber mat impregnated with thermoplastic Duroplast preproduct are then inserted and the inserted layers are pressed in the formed sheet-metal part and heated to convert them into a hardened material. Since the layer coated on the metal sheet has good adhesion to both the sheet-metal and the impregnated fiber mats, good adhesion of the layers to the sheet-metal part is assured. In this disclosure, however, no consideration is given to the difficulties that can arise when a hollow member must be assembled from several component sections by operations such as welding which require application of heat.